Graft loss from chronic rejection affects all solid organs to varying degrees and has become the major obstacle to the long-term success of heart transplantation. The applicants have established a clinically relevant large animal model of chronic heart transplant rejection, manifested by the development of cardiac allograft vasculopathy (CAV), using MHC inbred miniature swine. During the previous grant period, they characterized the effects of class I MHC incompatibilities on the pathogenesis of CAV. The current proposal is designed to extend these studies by elucidating cellular and molecular mechanisms of chronic rejection in class I disparate hearts. Their hypothesis, based on emerging rodent studies, is that CAV is mediated by an immune process initiated by CD4+ T cells, which recognize MHC antigens via the indirect pathway of allorecognition and require a co-stimulatory signal for full activation. The corollary is that protocols to inhibit CD4+ T cell activation and induce tolerance to the indirect pathway will most effectively prevent, interrupt, and/or reverse vasculopathy. They plan to use the MHC class I disparate model to: 1) investigate the role and mechanisms of indirect allorecognition and the development of CAV, 2) define the role of T cell co-stimulatory pathways in the pathogenesis of CAV, and 3) determine whether the induction of peripheral (by co-stimulatory blockade) or central tolerance (using mixed chimerism and thymoheart allografts) can prevent CAV. These studies should lead to a better understanding of the cellular and molecular mechanisms of chronic rejection in a clinically relevant experimental model and may lead to the development of new strategies to prevent or treat this process.